Double faced double storage capacity medium

ABSTRACT

A first holographic data storage device has a first set of holograms stored thereon. A second holographic data storage device has a second set of holograms stored thereon. An opaque layer is disposed between and attached to one side of the first and the second holographic data storage devices. In the case of double reflective diffractive recording, the opaque layer is not necessary.

FIELD OF THE INVENTION

The present invention generally relates to photonics data memorydevices. In particular, the present invention relates to a double-faceddiffractive holographic data storage device.

BACKGROUND OF THE INVENTION

There is a strong interest in high-capacity data storage systems withfast data access due to an ever-increasing demand for data storage.Limitations in the storage density of conventional magnetic memorydevices have led to considerable research in the field of opticalmemories. Holographic memories have been proposed to supersede theoptical disc (CD-ROMs and DVDs) as a high-capacity digital storagemedium. The high density and speed of holographic memory results fromthe use of three-dimensional recording and from the ability tosimultaneously read out an entire page of data. The principal advantagesof holographic memory are a higher information density, a shortrandom-access time, and a high information transmission rate.

While holographic data storage systems have not yet replaced current CDand DVD systems, many advances continue to be made which furtherincrease the potential of storage capacity of holographic memories. Thisincludes the use of various multiplexing techniques such as angle,wavelength, phase-code, fractal, peristrophic, and shift. However,previous methods for recording information in highly multiplexed volumeholographic elements, and for reading them out, have not provedsatisfactory in terms of throughput, crosstalk, and storage capacity.

It has also been proposed to use double-sided holographic data storagedevice. However, issues such as crosstalk between layers, speed of dataaccess and speed of access to the double diffractive holographic layerscontinue to challenge technological advances in this area.

Thus, it would be desirable to provide a diffractive holographic datastorage device, which increases storage capacity by utilizing doublelayers of the data storage device. Also, it would be desirable toprovide techniques for providing fast access to double sides and layersof a diffractive holographic data storage device. Furthermore, it willbe desirable to provide a diffractive holographic data storage device,that is compatible with the traditional HYDIF multiplexing technology.The compatibility is coming from the smart association of twodiffractive sides recorded with HYDIF process but allowing by animprovement a simultaneous simple reading of both faces. Thisassociation doubles the storage capacity and increase global readingspeed of stored data access.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a double-sided diffractiveholographic data storage device includes a first diffractive holographicdata storage device having a first set of holograms stored thereon and asecond diffractive holographic data storage device having a second setof holograms stored thereon. The first reflective hologram is formed bythe diffracted part of the reference beam processed by the first side ofthe double-side diffractive holographic data storage device. The storagedevice may be coated with polypeptide material. As an example, an opaquelayer is disposed between and attached to one side of the first andsecond diffractive holographic data storage devices. In accordance withanother aspect of the invention, an apparatus and method for reading adouble-sided diffractive holographic data storage device having firstand second reflective holograms stored on first and second sidesrespectively is provided. The apparatus includes a light source forgenerating a reference beam, a multi-scanning device for directing thereference beam incident on the first side of the double-sideddiffractive holographic data storage device at a predetermined angle,wherein a first reflective hologram is formed by the reference beamreflected from the first side of the double-sided diffractiveholographic data storage device. A detecting device is provided fordetecting the reference beam reflected from the double-sided diffractiveholographic data storage device. A rotating unit rotates thedouble-sided diffractive holographic data storage device into one of twopositions, wherein when the rotating unit is in a first position, thereference beam is incident upon the first side of the double-sideddiffractive holographic data storage device and the detecting devicedetects the first output data packet (i.e., diffractive holographicimage) reflected from the first side of the diffractive holographic datastorage device, and when the rotating unit is in a second position, thereference beam is incident upon a second side of a double-sideddiffractive holographic data storage device, and its detecting devicedetects the second output data packet (i.e., diffractive holographicimage) reflected from the second side of the diffractive holographicdata storage device.

According to another aspect of the invention, an apparatus and method ofreading a double-sided diffractive holographic data storage devicehaving a reflective and a transmissive set of diffractive patternsstoring data packets on first and second sides is provided.

Further objects, advantages, and novel features of the present inventionwill become apparent to those skilled in the art from this disclosure,including the following detailed description, as well as by practice ofthe invention. While the invention is described below with reference toa preferred embodiment(s), it should be understood that the invention isnot limited thereto. Those of ordinary skill in the art having access tothe teachings herein will recognize additional implementations,modifications, and embodiments, as well as other fields of use, whichare within the scope of the invention as disclosed and claimed hereinand with respect to which the invention could be of significant utility.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present invention,reference is now made to the appended drawings. These drawings shouldnot be construed as limiting the present invention, but are intended tobe exemplary only.

FIGS. 1A-1D are graphs illustrating recording and reading processes oftransmissive and reflective holography in accordance with one embodimentof the present invention.

FIG. 2 is a schematic representation of an apparatus for recording datain the form of a reflective hologram in accordance with one embodimentof the present invention.

FIG. 3 is a graph showing the effect of recording angle on angularselectivity in accordance with one embodiment of the present invention.

FIG. 4 is a schematic representation of a double-faced diffractiveholographic data storage device in accordance with one embodiment of theinvention.

FIG. 5 is a schematic representation of an apparatus for readinginformation stored on the double-faced diffractive holographic datastorage device shown in FIG. 4 in accordance with one embodiment of theinvention.

FIG. 6 is a schematic representation of an apparatus for readinginformation stored on the double-faced diffractive holographic datastorage device shown in FIG. 4 in accordance with another embodiment ofthe invention.

FIG. 7 is a schematic representation of an apparatus for readinginformation stored on the double-faced diffractive holographic datastorage device shown on FIG. 4 in accordance with yet another embodimentof the invention.

FIG. 8 is a schematic representation of an apparatus for readinginformation stored on the double-faced diffractive holographic datastorage device incorporating both reflective and transmissive hologramsin accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Storing/Recording Phase

A diffractive holographic data storage device contains informationstored during a phase of storing information. In the storing orrecording phase, a laser emits a coherent light beam that is split intotwo beams, a reference beam and an object beam, by means of a splitter(as shown in FIG. 2). The object beam may be filtered and collimated.The object beam is directed to a display means, which displays an imageto be recorded. The object beam becomes modulated by the information tobe recorded by means of reflection off the display or transmissionthrough the display.

The display may be any device for displaying a data packet in a system,such as a spatial light modulator (SLM) or liquid crystal light valve(LCLV). The plurality of bits represented on the display screen of thedisplay may be presented as a two-dimensional pattern of transparent andopaque pixels (i.e., data packet). The data packet displayed is derivedfrom any source such as a computer program, the Internet, and so forth.In an Internet storage application, the packets displayed may beformatted similarly to the packets of the Internet.

The reference laser beam defines the address where the information is tobe stored. The reference laser beam interferes coherently with theobject beam, which is the laser beam carrying the information to bestored, to form the interference pattern or hologram, which is stored inthe memory device due to the perturbation in the refractive index. Thus,each hologram is stored at a unique angle of the reference beam. Theseparation between the various holograms stored within the same volumerelies on the coherent nature of the hologram, in order to allow itsretrieval in phase with the volume only for a defined angle value. It isnoted that the reference beam may undergo various reflections andorientations using a set of mirrors to modify the angle between thereference beam and the object beam. Thus, by this mechanism angularmultiplexing is implemented. In other words, angular multiplexing iscarried out by sequentially changing the angle of the reference beam bymeans of mirrors. The multiplexing process may be programmable. It isalso contemplated that the reference beam provides an identity for thepage carried by the signal beam or object beam, so that the informationis distinguishable from other pages sharing the same volume inside thediffractive holographic data storage medium.

Whether a reflective holograph memory or a transmissive holograph memoryis produced depends on the recording process. A transmissive hologram isproduced when, in the recording process, the reference beam and objectbeam are on the same side of the diffractive holographic plate (FIG.1A). A reflective hologram is produced when, in the recording process,the reference beam and object beam are on the opposite side of thediffractive holographic plate (FIG. 1C). Details of a reading process ofthe different types of holograms are described below.

Reading Phase

Retrieving the stored information from the diffractive holographic datastorage device requires the use of a read beam whose characteristicscorrespond to those employed for writing or for storage (wavelength,angle of incidence and position within the storage material). This readbeam induces diffraction due to perturbation in the refractive indexcorresponding to the characteristics of the beam, thereby creating thestored modulated beam. The read beam carries the address of the pageselected for retrieval. Physically, addressing during retrieval issimilar to the recording phase (i.e., the read beam replicates thereference beam used for storing the desired pages).

The read beam may be controlled by an addressing-read system thatincludes mirrors or micromirrors associated with actuators, i.e.,galvanometers or micromotors, therefore capable of undergoing rotationthat allows each mirror to be oriented in the desired direction. Thesemirrors may be positioned at defined points or nodes by software, forthe purpose of angularly indexing a wavefront for a point of definedcoordinates (X,Y) in the memory device. The laser beam angularprocessing can be also implemented through dynamic means of grating oracoustic optics or a joint use of both or other microtechnologies.

In angular multiplexing, the read beam is positioned in order to accessa data packet contained at a defined point (X,Y) in a diffractiveholographic data storage device corresponding to an addressing angle.The reference beam angles in the reading procedure are similar to thereading (e.g., reference) beam angles the writing or recordingprocedure. However, the reading procedure may be carried out with agreater degree of tolerance than the recording procedure. It is possibleto use a very compact laser source of a solid-state type for the readingprocess because laser power necessary for reading is much lower than theone for recording. It is contemplated that the wavelength of the readbeam may be the same at the wavelength of the recording beam (e.g.,reference beam).

Referring to FIG. 1A, there is shown a schematic representation of atransmissive hologram in accordance with conventional holographicrecording techniques. In the recording set up, the object beam and thereference beam reach the recording plate on the same side. Referring toFIG. 1C, there is shown a schematic representation of a reflectivehologram in accordance with non-conventional holographic recordingtechniques. In this recording setup, the object beam and the referencebeam reach the recording plate on the opposite side. FIG. 1B isreferring to a schematic representation of a transmissive hologram inaccordance with diffractive holographic reading techniques. To read apage or a packet of information, the reading and output beams arelocated on the opposite side of the transmissive hologram, while FIG. 1Dis referring to a schematic representation of a reflective hologram inaccordance with diffractive holographic reading techniques. In thiscase, the reading and output beams are located on the same side of thereflective hologram.

Referring to FIG. 2, there is shown a schematic representation of anapparatus 200 for recording data in the form of a reflective hologramaccording to one embodiment of the invention. The recording apparatus200 includes a laser 220, a beam splitter 270, a tilting micromirror240, a multimirror device 260, a data recording plate 280, a first lens230, a second lens 210, a display device (e.g., spatial light modulator(SLM)) 250, mirrors 215, 225, and a computer 290.

In the recording apparatus 200 shown in FIG. 2, a light beam 201 fromthe laser 220 is directed to the splitter 270 which splits the lightbeam 201 into an object beam 202 and a reference beam 203. The referencebeam 203 is then emitted to the tilting micromirror unit 240, whichdirects the beam to a preselected mirror in the multimirror device 260.The light is reflected from the multimirror device 260 to a datarecording plate 280 which comprises a polypeptide material or othermaterials with similar characteristics. Further details of the angularmultiplexing technique used in the recording apparatus of 200 as well asthe polypeptide material in the recording plate 280 are described in acopending application entitled “Photonics Data Storage System Using aPolypeptide Material and Method for Making Same”, Serial No.PCT/FR01/02386, which is hereby incorporated by reference in itsentirety.

The polypeptide layer may be calibrated to resolve in a thickness rangeof approximately 10 to 40 micros depending on the application. It shouldbe noted that some crosstalk between the layers may limit the density ofeach layer so as to reduce the density of the optical density.Nevertheless, the two sides result in a doubling of the global densitywhich more than makes up for this loss. To optimize the crosstalk, eachlayer may be constructed using a different composition. For example,each layer may have different doping. It is noted that the underneathlayer receives less light energy than the above one layers and becauseevery layer absorbs one part of the energy, the underneath layer has theresponse. Therefore, the doping may be adapted to compensate fordifferent layers. The process is to be adapted for every layer and thatthrough the process, the top layer is more hardened because it supportsthe protective coat. Thus, a controlling process of the thickness ofevery layer may be developed to achieve optimization of the crosstalk.

The computer 290 generates data recorded with two consecutive angles,which is to be stored on the data recording plate 280. This data istransferred to an optical representation on the SLM 250. The object beamlight 202 reflects off mirrors 215 and 225 and passes through the SLM250. After passing through the SLM 250, the light is modulated andreaches lenses 230 and 210 which collimate the light and direct it tothe back of the data recording plate 280, forming a reflectivediffractive holographic image by interference between the reference beam203 reflected from the multimirror device 260.

Referring to FIG. 3, a graph is provided showing the effect of recordingangle on angular selectivity according to one embodiment of theinvention. Angular selectivity is defined as the angle of separation,which is required to prevent crosstalk between two adjacent packets ofdata in an angularly multiplexed hologram. The graph shows that for thesame thickness of a polypeptide layer the angular selectivity is between1° and 6° for a transmissive hologram and less 1° for a reflectivehologram.

The angle selectivity ΔΘ may be different in the reflective case and inthe transmissive case, the reason being that the physics of the layerinternal molecular organization induced by light modulation in the twocases is not the same. The angular selectivity is defined as:ΔΘ=λ/2d sin(Θ_(B))

-   -   Where ΔΘ is the angular difference between two angular        multiplexing angles;    -   d is the thickness of the polypeptide layer; and    -   Θ_(B) is the Bragg angle.

In is contemplated that this angle, for a given modulation, givesmaximum diffraction efficiency. In one embodiment, this angle can be therecording angle in the case where there is no modification of thethickness of the polypeptide layer between recording and reading.

FIG. 4 is a schematic representation of a double-faced diffractiveholographic data storage device in accordance with one embodiment of theinvention. The double-faced diffractive holographic data storage device400 includes a first face 410, a second face 420 and an opaque medium430.

Each of the 410 and 420 faces is recorded using the recording apparatusas shown in FIG. 1. Each separate diffractive holographic data storagedevice is then attached to opposite sides of an opaque medium 430. Inone embodiment, these separate diffractive holographic memories arereflective holograms. The reflective hologram is formed when the objectbeam and the reference beam are located at the opposite side of therecording plate during the recording process. The opaque medium 430 isused to prevent light from coming from one side of the reading beam andreading the second side in the reading process. The opaque medium 430may include a photosensitive layer that is coated between the glasssubstrates and darkened after UV (ultraviolet) light.

FIG. 5 is a schematic representation of an apparatus for readinginformation stored on the double-sided diffractive holographic datastorage device as shown in FIG. 4 according to one embodiment of theinvention. The apparatus 500 includes a laser 538, a multi-scanningdevice 540, a detector 546, imaging lenses 542 and 544, a rotating table550, and the double-faced memory unit 400.

The laser 538 generates a beam of light, which is directed by amulti-scanning device 540 to the double-faced memory unit 400. Asdescribed in FIG. 4, the double-faced memory unit 400 includes first andsecond faces 410 and 420. One of the reflective holograms from thedouble-faced memory unit 400 passes through the pair of lenses 542, 544before reaching the detector 546. The first side (i.e., face) 410 of thedouble-faced memory unit 400 is read in the configuration shown. Inorder to read the opposite side 420 of the double-faced memory unit 400,the rotating table 550 is rotated at an angle (e.g., 180° so that thebeam from the multi-scanning device 540 reaches that second face 420.The reading process of information on the second face 420 is similar tothe reading process on the first face 410.

FIG. 6 is a schematic representation of the apparatus for reading thedouble-faced memory unit 400 according to another embodiment of thepresent invention. The apparatus 600 includes a detector multi-scanningdevice 640, the double-faced memory unit 400, a detecting system 610,and a pair of lenses 642 and 644.

The multi-scanning device 640 includes a tilting micromirror 650 and amultimirror device 652. The detecting system 610 comprises a CCD(charge-coupled device), e.g., camera 646 coupled to a computer 654 anda monitor 656. The reading process applied in this embodiment is similarto the reading process described in FIG. 5 in which the reading of oneside of the recording plate 400 occurs before the reading of the otherside of the recording plate.

Referring now to FIG. 7, another embodiment of an apparatus for readinga double-faced memory unit 400 is shown. The apparatus 700 includes alaser 758, a beam splitter 760, mirrors 761 and 762, two multi-scanningdevices 764 and 768, the double-faced memory unit 400, two pairs ofimaging lenses 772, 770 and 776 and 778, and two detectors 774 and 780.

This apparatus 700 eliminates the necessity of a rotating table (e.g.,rotating table 650 shown in FIG. 6) and thus greatly increases the speedof the reading process since the reading of the double-faced memory unit400 is simultaneous. In other words, both sides of the double-facedmemory 400 can be read in parallel with this implementation.

The beam splitter 760 receives a light beam from the laser 758 andperforms a splitting function to the light beam. The mirror 761 and 762each directs a portion of the beam from the laser 758 to the first andsecond multi-scanning devices 764 and 768, respectively. Themulti-scanning devices 764, 768 each direct a beam to a respective firstand second side 410, 420 of the double-faced diffractive holographicdata storage device 400. The resulting hologram from the first side 410passes through imaging lenses 770 and 772 and is detected by detector774. Additionally, the resulting holograph from the second side 420 isdirected to a pair of lenses 776 and 778 and is directed by detector780.

FIG. 8 is a schematic representation of an apparatus for readinginformation stored on the double-faced diffractive holographic datastorage device incorporating both reflective and transmissive hologramsin accordance with one embodiment of the present invention. Theapparatus 800 includes a double-faced hologram memory device 810, adetector 820, and a detector 830. The memory device 810 includes amemory component 845 and a memory component 855. The two holograms arebonded together in such a manner that in a reading setup simultaneousaccess of both components 845 and 855 is achieved. When the referencebeam emits upon the component 845, a first hologram is generated and afirst output is detected by the detector 820. The same reference beampasses through component 845 and reaches component 855 to generate asecond hologram and a second output page is detected from the hologramby detector 830. A portion of the reading beam diffracted by themultiplexed volume hologram forms the reconstruction, which is detectedby the detectors 820 and 830. The reconstructed beam carries the datathat is a replica of the desired page. It is contemplated that there isnot an opaque medium between the two components 845 and 855 so thatsimultaneous reading can be achieved. In order to read information fromboth sides of the combined memory device 810, component 845 is areflective hologram and component 855 is a transmissive hologram.

The present invention is not to be limited in scope by the specificembodiments described herein. Indeed, this application is intended tocover any modifications of the present invention, in addition to thosedescribed herein, and the present invention is not confined to thedetails which have been set forth. Thus, the scope of the inventionshould be determined by the appended claims and their legal equivalents,rather than by the examples given.

1. An apparatus comprising: a first diffractive holographic data storagedevice having a first set of holograms stored thereon; a seconddiffractive holographic data storage device having a second set ofholograms stored thereon; and an opaque layer disposed between andattached to one side of the first and second diffractive holographicdata storage devices.
 2. The apparatus according to claim 1 wherein thefirst and second diffractive holographic data storage devices arereflective holograms.
 3. The apparatus according to claim 2 wherein thefirst and second diffractive holographic data storage devices includemultiplexed holograms.
 4. The apparatus according to claim 3 wherein themultiplexed holograms are angularly multiplexed.
 5. The apparatusaccording to claim 1 wherein the first and second diffractiveholographic data storage devices include an organic material.
 6. Theapparatus according to claim 5 wherein the organic material is apolypeptide material.
 7. An apparatus comprising: a reflectivediffractive holographic data storage device having a first set ofholograms stored thereon; and a transmissive diffractive holographicdata storage device having a second set of holograms stored thereon, thetransmissive and reflective diffractive holographic data storage devicesbeing attached together.
 8. An apparatus for reading a double-sideddiffractive holographic data storage device having first and secondreflective holograms stored on first and second sides respectively,comprising: a multi-scanning device for directing a reference beamincident on one of the first and second sides at a predetermined angle;and a detecting device for detecting the reference beam reflected fromthe storage device.
 9. The apparatus according to claim 8 furthercomprising a rotating unit for rotating the double-sided diffractiveholographic data storage device into first and second positions.
 10. Theapparatus according to claim 9 wherein the rotating unit is in the firstposition when the reference beam is incident upon the first side and thedetecting device detects a first diffractive data output packetsproduced by reflective diffraction from the first side.
 11. Theapparatus according to claim 9 wherein the rotating unit is in thesecond position when the reference beam is incident upon the second sideand the detecting device detects a second data packet output produced byreflective diffraction from the second side.
 12. The apparatus accordingto claim 8 wherein the first and second reflective holograms areangularly multiplexed holograms.
 13. The apparatus according to claim 8wherein the double-sided device includes an organic material.
 14. Theapparatus according to claim 13 wherein the organic material is apolypeptide.
 15. The apparatus according to claim 8 wherein thereference beam is coherent or incoherent.
 16. An apparatus for readingdual layer located on a diffractive device comprising: a firstmulti-scanning device for directing a first read beam incident upon afirst side of the diffractive device at a first predetermined angle; asecond multi-scanning device for directing a second read beam incidentupon a second side of the diffractive device at a second predeterminedangle; a first detecting device for detecting a first diffractive packetdata output formed by the first reflectively diffracted read beamreflectively diffracted from the first side; and a second detectingdevice for detecting a second diffractive holographic image formed bythe second reflectively diffractive read beam reflected from the secondside.
 17. The apparatus according to claim 16 wherein the first readbeam is generated from a coherent or non-coherent light having a samewavelength as a recording light.
 18. The apparatus according to claim 16wherein the second read beam is coming from a laser or a portion of thefirst read beam is coming through a beam splitter.
 19. The apparatusaccording to claim 16 wherein the dual layer located on double-facedplate is an angularly multiplexed hologram.
 20. The apparatus accordingto claim 16 wherein the reference beam is coherent or incoherent.
 21. Anapparatus comprising: a diffractive holographic data storage devicehaving a first side with a reflective hologram stored thereon and asecond side with a transmissive hologram stored thereon; amulti-scanning device for directing a read beam incident on the firstside, wherein a first portion of the read beam forms a first diffractiveholographic image, wherein the image is produced by reflectivediffraction from the first side, and wherein a second portion of theread beam is transmitted through the diffractive holographic datastorage device and forms a second diffractive holographic image; a firstdetector for detecting output data packet produced by reflectivediffraction from the first side; and a second detector for detecting theoutput data packet produced by transmission diffraction from the secondside.
 22. The apparatus according to claim 21 wherein the read beam isgenerated from a coherent light source.
 23. The apparatus according toclaim 21 wherein the first output data packet is generated by reflectivediffraction from the first side reflective holography.
 24. The apparatusaccording to claim 21 wherein the second output data packet is generatedby transmissive diffraction from the second side through transmissiveholography.
 25. A holographic device comprising: a first holographicunit having a first front surface and a first back surface wherein areflective hologram is formed by interference between object andreference beams on the first front surface; and a second holographicunit having a second front surface and a second back surface wherein atransmissive hologram is formed on the second front surface, the secondfront surface being attached to the first back surface.
 26. Theapparatus according to claim 25 wherein a reference beam is directed tothe first front surface to generate the reflective hologram reflectivelydiffracted from the first surface, and the transmissive hologram istransmitted through the second holographic unit and emitted out of thesecond back surface.
 27. The apparatus according to claim 26 wherein thereflective hologram is formed by object and reference beams directed tothe first front surface and the transmissive hologram is formed by thereference beam directed to the second front surface and the object beamdirected to the second back surface.
 28. The apparatus according toclaim 24 wherein the first and second holographic units each comprise atleast two layers, the first layer comprising a thin film of holographicrecording material and the second layer comprising a substrate.
 29. Theapparatus according to claim 28 wherein the holographic recordingmaterial is an organic material.
 30. The apparatus according to claim 29wherein the organic material is a polypeptide.
 31. The apparatusaccording to claim 25 wherein the first or second holographic device isangularly multiplexed.
 32. A method comprising: providing a firstholographic data storage device having a first set of holograms storedthereon; providing a second holographic data storage device having asecond set of holograms stored thereon; disposing an opaque layerbetween the first and second holographic data storage devices; andattaching the opaque layer to one side of the first and secondholographic data storage devices.
 33. The method according to claim 32wherein the first and second holographic data storage devices arereflective holograms.
 34. The method according to claim 33 wherein thefirst and second holographic data storage devices include multiplexedholograms.
 35. The method according to claim 34 wherein the multiplexedholograms are angularly multiplexed.
 36. The method according to claim32 wherein the first and second holographic data storage devicesincludes an organic material.
 37. The method according to claim 36wherein the organic material is a polypeptide material.
 38. The methodaccording to claim 32 wherein the first holographic data storage deviceis a reflective hologram and the second is a transmissive hologram. 39.A method for reading a double-sided holographic data storage devicehaving first and second reflective holograms stored on first and secondsides respectively, comprising: directing a reference beam incident onone of the first and second sides at a predetermined angle; anddetecting the reference beam reflectively diffracted from the storagedevice.
 40. The method according to claim 39 further comprising rotatingthe double-sided diffractive holographic data storage device into firstand second positions.
 41. The method according to claim 40 wherein therotating is in the first position when the reference beam is incident onthe first side and the detecting device detects a first output datapacket produced by reflective diffraction from the first side.
 42. Themethod according to claim 40 wherein the rotating unit is in the secondposition when the reference beam is incident upon the second side andthe detecting device detects a second holographic image reflectivelydiffracted from the second side of the holographic storage device. 43.The method according to claim 39 wherein the first and second reflectiveholograms are angularly multiplexed holograms.
 44. The method accordingto claim 39 wherein the double-sided device includes an organicmaterial.
 45. The method according to claim 39 wherein the organicmaterial is a polypeptide.
 46. The method according to claim 39 whereinthe reference beam is coherent or incoherent light beam.
 47. A methodfor reading a double-sided hologram comprising: directing a first readbeam incident upon a first side of the hologram at a first predeterminedangle; directing a second read beam incident upon a second side of thehologram at a second predetermined angle; detecting a first output datapacket produced by reflective diffraction of the first read beam fromthe first side; and detecting a second output data packet produced bythe reflective diffraction of the second read beam from the second side.48. The method according to claim 47 further comprising generating thefirst read beam from a light source having a same wavelength as arecording wavelength.
 49. The method according to claim 47 furthercomprising generating the second read beam, the second read beam being aportion of the first read beam.
 50. The method according to claim 47wherein the double-sided hologram is an angularly multiplexed hologram.51. The method according to claim 47 wherein the first read beam is acoherent or incoherent light beam.
 52. A method for reading datacomprising: providing a holographic data storage device having a firstside with a reflective hologram stored thereon and a second side with atransmissive hologram stored thereon; directing a read beam incident onthe first side, wherein a first portion of the read beam forms a firstoutput data packet produced by reflective diffraction from the firstside, and wherein a second portion of the read beam is transmittedthrough the holographic data storage device and forms a second outputdata packet; detecting the output data packet produced by reflectivediffraction; and detecting the output data packet produced bytransmissive diffraction from the second side.
 53. The method accordingto claim 52 wherein the read beam is generated from a coherent lightsource.
 54. The method according to claim 52 wherein the firstdiffractive output data packet is generated by the reflective hologram.55. The method according to claim 52 wherein the second diffractiveoutput data packet is generated by the transmissive hologram.
 56. Amethod comprising: providing a first holographic unit having a firstfront surface and a first back surface wherein a reflective hologram isformed on the first front surface; and providing a second holographicunit having a second front surface and a second back surface wherein atransmissive hologram is formed on the second front surface; andattaching the second front surface to the first back surface of thefirst holographic unit.
 57. The method according to claim 56 wherein areference beam is directed to the first front surface to generate thereflective hologram reflected from the first surface, and the secondhologram is transmitted through the second holographic unit and emittedout of the second back surface.
 58. The method according to claim 57wherein the reflective hologram is formed by object and reference beamsdirected to the first front surface and the transmissive hologram isformed by the reference beam directed to the second front surface andthe object beam directed to the second back surface.
 59. The methodaccording to claim 55 wherein the first and second holographic unitseach comprise at least two layers, the first layer comprising a thinfilm of holographic recording material and the second layer comprising asubstrate.
 60. The method according to claim 58 wherein the holographicrecording material is an organic material.
 61. The method according toclaim 69 wherein the organic material is a polypeptide.